Physical properties of a sample can be determined by interpreting measured optical characteristics of the sample. For example, the optical characteristics may describe the light that has scattered from the sample, given the description of the light incident upon the sample. Physical properties of particular interest are parameters of grating-like structures on a silicon wafer. A reflectometer operating at near normal incidence is one example of an optical instrument that can be used to measure the properties of gratings on a wafer. In general, the interpretation mentioned above either implicitly or explicitly compares measured light intensities to the predictions of an optical model, which describes the incident light, the optical characteristics of the sample, and the detection of light.
It is desirable in many situations to allow the wafer to be viewed at any rotational orientation upon its support. Allowance for arbitrary rotation of the sample is desired, for example, if the optical instrument is integrated into a process tool like a lithography track or polishing tool for chemical mechanical polishing. A robot transports wafers (particular samples of interest) within the process tool to various process modules, and also delivers wafers to the metrology system, which contains the optical instrument. The wafer is typically placed on a flat support. The process tool as a whole may not be sensitive to the specific rotation of the wafer at any point, and may have no provision for determining that orientation. Even if the orientation of the wafer is determined at some point in its processing path through the process tool, the process modules or the robot may not maintain this orientation. Since space is typically at a premium in such a process tool, it is preferable to not need an independent “wafer aligner” for the metrology instrument.
The optical characteristics of grating-like structures have a marked sensitivity to the polarization of light. Samples with grating-like structures will affect the amplitude and phase of the light they reflect or transmit differently for different incident polarizations. The same is also true for birefringent samples, or stacks of thin films at other than normal incidence. This can be an issue when making measurements with some photometric instruments. In lithography applications, for example, determining the linewidth or profile of diffractive pattern features formed on a semiconductor wafer or photomask may be performed by measuring the normal or near-normal incidence (hereafter collectively referred to as quasi-normal incidence) reflectivity or other optical properties with a small-spot reflectometer or small-spot transmissive spectrophotometer. The spectral reflectivity or transmissivity of the sample being measured will depend to some extent on the degree of polarization of the incident light and on the orientation of the wafer. Thus, in order to allow arbitrary orientation of a grating sample whose optical characteristics depend strongly on polarization of the light, the illumination by the metrology instrument must be effectively unpolarized. The detection by the instrument must likewise be insensitive to polarization.
In some instruments it is possible to orient the sample so that the grating-like structures of the pattern (or the optical axis of a birefringent surface or thin film stack) are presented in a known and consistent direction relative to the instrument's incident light. Any systematic errors due to polarization can then be minimized during data processing. That is, by carefully characterizing the polarization characteristics of the optics and modeling the effect on a sample's response at a particular sample orientation relative to the polarized light, the measured data can be processed so as to eliminate the polarization effect provided the sample is measured at the modeled orientation.
However, it is not always possible to provide a specified sample orientation to the measuring instrument. Wafer handlers associated with lithography tracks frequently present the samples to the measuring instrument in a consistent but unknown orientation that the measuring instrument itself has no control over. Polishers produce a random sample orientation. Hence, it would be preferable if the instrument's illumination and collection optics were non-polarizing, so that orienting the wafer would be unnecessary.
In the past, the effect of instrument polarization on measurement results have been only a minor issue that has typically been ignored except in those instruments where polarization itself is the parameter being measured. Polarimeters and ellipsometers deliberately use incident light of known polarization. Also, until recently, spectrometry instruments were not used for measuring linewidth, profile, etc. of grating-like structures.
Unwanted polarization in the optics can be caused by polarizing elements such as tilted fold mirrors, beamsplitters, tilted glass surfaces, prisms, and spectrometer gratings. (In this context “polarizing” can mean partially polarizing or in some way affecting the polarization state.) One prior solution has been to reduce the polarization effect of instrument components by carefully arranging the planes of incidence of the tilted components in the system, so that for every such tilted component the instrument also has a similar component tilted in the perpendicular plane to cancel the polarization effect of the first. This use of component pairs requires more room for the optics, so that it cannot be used when a compact system is needed. The pairing technique cannot be used to alleviate the polarization effect in the spectrometer component of the system.
Depolarizers of several types are known. In Zeiss monolithic spectrometers, among others, light is coupled with a fiberoptic bundle that scrambles the polarization. Fiber depolarizers cannot be used in the imaging path because they would also scramble information about the image. Wedge depolarizers, comprising a birefringent wedge plate and an index-matched non-birefringent plate, need to be properly oriented to the polarization of the light to be depolarized. Because they produce a laterally offset double image, they are not well suited for imaging systems.
Lyot depolarizers, comprising two non-wedge-shaped birefringent plates with their axes at 45° to each other, are commercially available, for example from Karl Lambrecht and other optical component manufacturers. The basic element of a (plate) Lyot depolarizer, as shown in FIG. 1, is a birefringent plate 1 with “retardance” d. The retardance is given by
                    d        =                                                            2                ⁢                                                                  ⁢                π                            λ                        ⁢                          (                                                n                  o                                -                                  n                  e                                            )                        ⁢            t                    =                      2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                          kf              .                                                          Eq        .                                  ⁢        1            wherein λ is the wavelength in vacuum, t is the thickness of the plate, no is the optical index of the ordinary axis 3, ne is the optical index of the extraordinary axis 5, k is the wavenumber (in vacuum), and f is the “retardance frequency”: the frequency (i.e., reciprocal period) of oscillations of the optical response of the plate as a function of wavenumber,
                    k        =                              2            ⁢                                                  ⁢            π                    λ                                    Eq        .                                  ⁢        2                                f        =                                            (                                                n                  o                                -                                  n                  e                                            )                        ⁢            t                                2            ⁢                                                  ⁢            π                                              Eq        .                                  ⁢        3            (f is not strictly constant with respect to wavelength because no and ne are typically wavelength-dependent, but the wavelength variation of f is typically much smaller than its magnitude.) Fiducial line 7 is for illustrative purposes to indicate the position of the ordinary axis. The frequency of polarization variations induced by the plate is proportional to thickness of the plate and the difference between ordinary and extraordinary indices.
As shown in FIG. 2, and described in U.S. Pat. No. 5,371,595, a Lyot depolarizer 11 consists of two birefringent plates 13 and 15 with retardance frequencies in the ratio of 1:2, and with a relative rotation 17 of 45° (π/4 radians) between their polarization axes. If the two plates are of the same material, the thicknesses will also be in the ratio of 1:2. The thinner plate will have the lower retardance frequency f0 corresponding to retardance d. The thicker will have retardance frequency 2f0 corresponding to retardance 2d. The thinner plate is typically about 2 millimeters thick. Incident light 19 passing through the Lyot depolarizer 11 and emerging as transmitted light 21 has its polarization scrambled in a wavelength-dependent manner.
Lyot depolarizers have previously been used in imaging spectroradiometers and spectropolarimeters for telescopes, for example on a satellite observing backscattered radiation from the earth to monitor atmospheric ozone depletion. In contrast to fiber and wedge depolarizers, Lyot depolarizers are image-preserving, and are therefore suitable for imaging systems.
An object of the present invention is to provide a small-spot spectrometry instrument with pattern viewing capability for measuring grating-like or other diffractive pattern structures on semiconductor wafers, photomasks, and the like, wherein the instrument's polarization effects on linewidth, profile, erosion and similar feature measurements are minimized.
Another object of the present invention is to provide a depolarizer that scrambles the polarization as a function of wavelength with improved characteristics, e.g., over a Lyot depolarizer.
An additional object of the present invention is to provide a spectroscopy instrument that behaves as an ideal unpolarized instrument through the use of such an improved depolarizer.